The corticotropin-releasing factor (or hormone) of the hypothalamus will be purified from tissue of several species. The course of the purification will be followed by the ability of preparations to stimulate the release of ACTH from pituitary tissue in vitro. The ACTH will be measured either by in vitro bioassay or by radioimmunoassay. CRF will be extracted from tissues with dilute acid and concentrated by fractional precipitation with acetone. Further steps will involve gel filtration, ion exchange chromatography, partition chromatography and electrophoresis. The CRF will be purified to constant specifc activity and will be subjected to amino acid analysis and sequencing. Purified CRF will be tested for direct effects on other than pituitary tissues. If sufficient CRF can be obtained, we shall try to develop a radioimmunoassay.